FIG. 1 shows a conventional monochrome OLED device 100. The OLED device comprises a functional stack of one or more organic functional layers 104 between a conductive cathode layer 102 and an anode layer 106. The functional stack is formed on a transparent substrate 108. The conductive layers can be patterned to form one or more cells or pixels on the substrate. In operation, charge carriers are injected through the cathodes and anodes for recombination in the functional layers. The recombination of the charge carriers causes the functional layer to emit visible radiation.
The basic advantages of OLED devices are low driving voltage, low power consumption, large viewing angle, high contrast, fast response, rugged design and the potential for low manufacturing costs. To fully utilize these advantages of OLED devices and for potential applicability to color flat panel displays, multi color and full color versions can be produced.
There are several methods to fabricate color displays: a broadband (i.e. white) emitter and color filters, a short wavelength (i.e. blue) emitter and down-converter for green and red emitters, organic materials which emit different colors on the same substrate, and microcavity structures for spectral filtering, masked diffusion of dye materials or selective laser bleaching of dyes.
All of these methods suffer from drawbacks related to material unavailability, power inefficiency, image distortion, costly and inefficient fabrication, reduced performance and other reasons. For example, there is hardly any efficient and stable broadband emitter available. The use of color conversion materials results in image distortions and less brilliant displays. Generating pixels on a single substrate surface with different light emitting organic materials usually results in loss in device performance and spatial resolution. Ink-jet printing is one approach but it is essentially a serial process that results in low resolution and low production throughput.
Another approach is to stack layers of different emitting heterostructure devices on a single substrate surface. One known example of a multicolor OLED device employing a vertical stacked structure is disclosed in Forrest et al, U.S. Patent Application 20010000005. The main drawback is that the materials used for the stacked structure have to be substantially transparent with low optical absorption, since the beam of light from each device is wholly coincident with the others, hence imposing many constraints on the device architecture.
As evidenced from the discussion above, it is desirable to provide an improved fabrication process designed for efficient production of multi and full color OLED devices.